1. Field of the Invention
The present invention relates generally to a control system for an automatic power transmission of an automotive vehicle. More specifically, the invention relates to a shifting transition control in the automatic power transmission for optimization of shifting transition.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 57-144338 discloses one example of an automotive automatic power transmission. The shown transmission controls transmission speed ratio by supplying fluid pressure for friction elements depending upon a throttle valve open angle and a vehicle speed. The fluid pressure acting on the friction elements is adjusted depending upon the throttle valve open angle. Therefore, the fluid pressure in the friction element is also controlled depending upon the throttle valve open angle.
Such prior proposed system encounters a drawback in fluctuation of shifting performance, such as magnitude of shift shock and so forth. Furthermore, such prior proposed system may be subject a problem of secular variation of the shifting performance due to tolerance in valve, spring and other transmission components, difference of hydraulic characteristics of fluid circuit, difference of frictional characteristics of friction elements. Thus, an individual transmission may have a unique shifting performance. Furthermore, even when the initial shifting performance is suitably adjusted, the magnitude of shift shock can be increased due to a variation of the frictional characteristics of the friction elements due to wearing or due to fatigue of the working fluid.
In order to solve such drawback in the conventional automatic power transmission, Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-92863 discloses a shift control system, in which a fluid pressure acting on the friction elements is adjusted so as to adjust shifting transition period toward a predetermined period. The shown control system includes an engine load sensor for monitoring an engine load, a vehicle speed sensor for monitoring the vehicle speed, and a shifting commanding means which compares the engine load and the vehicle speed with a preset shifting pattern for detecting the vehicle driving condition requiring shifting of the transmission speed ratio and generating a shifting command. The system also includes a shift actuator which controls supply of fluid pressure for the friction elements depending upon commanded speed ratio, a fluid pressure commanding means responsive to the shifting command for commanding fluid pressure during shifting transition, a fluid pressure adjusting actuator responsive to a fluid pressure command for fluid pressure to be supplied to the friction elements, a shifting transition period measuring means for measuring an elapsed period from occurrence of the shifting command to completion of shifting operation, a shifting period memory for storing a most recent shifting transition period measured by the shifting transition period measuring means, a shifting transition period setting means which stores a set shifting transition period, a comparing means for comparing the shifting transition period stored in the shifting period memory and the set shifting transition period, and a fluid pressure correcting means which adjusts the fluid pressure to be supplied to the friction element so that the difference between the actual shifting transition period stored in the shifting period memory and the set shifting transition period is adjusted. With the shown construction, the shifting transition period can be successfully adjusted toward the set shifting transition period.
However, in such prior proposed automatic transmission control system, a drawback is still encountered. That is, since the prior proposed system adjusts the shifting transition period toward a singular set shifting transition period, the shifting transition period is uniformly controlled to the set period. For this, it should be appreciated that the optimal shifting transition period may be variable depending upon the vehicle driving condition, such as a vehicle speed. Namely, the greater energy absorption will be caused at a higher vehicle speed. That implies that a higher vehicle speed will require a longer shifting transition period for providing a longer period for greater energy absorption. In other words, when the shifting transition period is set for obtaining optimal shifting characteristics at a relatively low speed, the energy absorption rate in a unit time become greater at a relatively high vehicle speed range for causing a stronger shift shock. Furthermore, a high absorption rate of energy may cause a higher rate of wearing in the friction element and thus can shorten the life thereof. Furthermore, in the worse case, the heat generation rate by energy absorption may become excessive relative to the thermal capacities of the friction elements.